
In our FNDMC calculations, we adopted Slater-Jastrow type wavefunctions
as their fixed-node trial wavefunctions [1]}.
Molecular orbitals entering the single Slater determinants were
generated by DFT-B3LYP with various types of basis sets.
The Jastrow factor consists of one-, two-, and three-body
terms [2]} including 88 variational parameters in total.
These parameters were optimised through a variance minimisation scheme [3]}, and
only the two-body term holds the electron-electron cusp condition [4]}.
The electron-nucleus cusp condition, which is a short-range one-body correlation effect,
is satisfied by the cusp correction scheme applied to the Gaussian basis sets [5]}
instead of imposing the cusp condition on the one-body term.
Note that the Jastrow factor does not change the (fixed) nodal surfaces
and is responsible for the numerical stability in FNDMC.
However, the quality of the nodal surfaces determines
the accuracy of the FNDMC energies
in terms of the fixed-node variational principles [6]}.
Accordingly, the FNDMC accuracy depends implicitly on the basis set adopted,
which is used to expand the molecular orbitals entering the Slater determinant.
In addition to the fixed-node error, another source of bias
in actual FNDMC calculations arises from the short-time approximation
with finite (small) timesteps [7]}.
To remove this bias, it is common to use multiple timesteps
to make the linear regression and obtain
calculation results for \(\delta t \rightarrow 0\) .
This regression is an approximation of the theoretical \(\delta t = 0\)  result.
The present study considers both linear and quadratic extrapolations.
